Pesky downy mildews

Basil downy mildew spores can be seen on the bottom of an organic basil leaf. The spores give the leaf a purplish tinge.

The cooler weather of fall means conditions are right for downy mildew diseases to develop. Besides the downy mildew that attacks cucumbers, which I have mentioned in previous columns, four other downy mildews might show up in Lowcountry gardens.

Downy mildew is one type of water mold, algal-like organisms that attack and live on plants. They are very particular about which plants they infect.

Three different downy mildews attack plants in the mint family: basil, coleus and salvia. Even though these three plants are closely related, like cousins, the downy mildews on them are specialized to attack their “favorite” host plants and do not spread to the other types.

In 2007, basil downy mildew was brought inadvertently from Europe to Florida in contaminated basil seed. Since then, basil growers all along the East Coast have had to battle this disease in order to grow healthy sweet basil.

Basil downy mildew is common in the Lowcountry in the fall. Leaves turn yellow and then drop off. Diseased leaves still attached to the plant should not be eaten. Basil downy mildew can grow so thickly that the underside of green basil leaves appears purplish with downy mildew spores.

Like downy mildew on cucumber, basil downy mildew survives year-round in southern Florida. Spores are spread by wind to basil in states north of Florida.

Potassium phosphite is a biopesticide that controls basil downy mildew. It works best when sprayed before plants become infected. Cultivars of spice basil, like ‘Blue Spice,’ appear to be resistant.

Salvia downy mildew is found on certain types of salvias or sages. It was seen recently in the Lowcountry on the hybrid salvia ‘Wendy’s Wish.’ Downy mildew was reported from other states on mealy cup sage (Salvia farinacea), annual bedding salvia (Salvia splendens), and culinary sage (Salvia officinalis).

Leaf spots on mealy cup sage are dark brown, but on the other salvias, the symptoms are the more typical indistinct yellowing. Like other downy mildews, salvia downy mildew produces grayish spores on the bottom of leaves. Affected leaves will drop from the plant.

Another member of the large mint family that has its own downy mildew is coleus. Coleus downy mildew was first discovered in Louisiana and New York in 2005. It is still a problem in some areas of the country.

Like downy mildew on basil and salvia, coleus leaves get spots and fall off when they are infected with downy mildew. The spores on the bottom of the leaf are gray.

Coleus downy mildew prefers what we in the Lowcountry consider cool temperatures, below 70 degrees. Coleus downy mildew can’t develop at 85 degrees. Thus, if coleus are planted late in the spring and removed in early fall, it is unlikely that they will be affected by downy mildew.

If coleus need to be sprayed for downy mildew, the fungicide mancozeb is available to home gardeners.

Another type of downy mildew, this time on impatiens, has been in the U.S. since 2008, when it was found in New York. Impatiens downy mildew has made it difficult for homeowners to grow impatiens, a favorite summer bedding plant for shady gardens.

Impatiens respond to downy mildew by dropping most leaves. White spores are found on the bottom of the leaves that remain on the plant.

Impatiens growers have cut back on the number of impatiens they produce due to downy mildew. An alternative to impatiens (Impatiens walleriana) is New Guinea impatiens (hybrids of Impatiens hawkeri), which are resistant to downy mildew.

Impatiens downy mildew also survives over winter in Florida. However, this downy mildew likely can survive in soil in northerly locations. If leaves with downy mildew fall to the ground, it is possible that impatiens planted there the following year could get downy mildew again.

Impatiens can be treated with potassium phosphite or mancozeb, but success is not as likely as with the other downy mildews.

Managing downy mildews on basil, salvia, coleus and impatiens requires a keen eye to find symptoms early and prompt follow through if the disease is found.

Since most of these plants are annuals, it may be simplest to remove diseased plants and replace with something different.

Anthony Keinath is professor of plant pathology at the Clemson Coastal Research & Education Center in Charleston. His expertise is in diseases of vegetables. He is also an avid gardener. Contact him at tknth@ clemson.edu